A fluorine-containing polymer has been preferably used as a coating material applied to a surface of metal because of its excellent characteristics such as chemical resistance, thermal resistance and non-sticking property, and is used, for example, for lining of chemical apparatuses requiring corrosion resistance, inner lining of cooking apparatuses such as a rice cooker requiring corrosion resistance and non-sticking property, and other applications. However since its adhesion to a surface of metal is not sufficient due to its excellent non-sticking property, hitherto various modifications have been made to improve adhesion to the surface of metal.
One of them is a method for adhering a fluorine-containing polymer to a substrate by roughening the surface of metal chemically or physically with expecting an "anchor effect" therebetween. In this method, the surface roughening itself requires much labor, and though initial adhesion is possible, lowering of the anchor effect arises at the time of temperature change and at high temperature.
Also a method for chemically activating a surface of a fluorine-containing resin by treating the surface with a solution prepared by dissolving metallic sodium in liquid ammonia has been proposed. However, in this method, not only there was a fear that the solution itself causes environmental pollution but also there was a problem that its handling is attended with danger. Further, there have been proposed methods for carrying out physical and chemical treatment such as plasma sputtering on a surface of a fluorine-containing resin or mechanically roughening the surface of a fluorine-containing resin. However these methods have problems such that the mentioned treatments require much labor, increase in cost arises and design or appearance of a coated surface is injured.
Also in order to improve adhesion of a fluorine-containing coating, investigations with respect to addition of various components and use of a primer have been made.
For example, there is a technique of adding an inorganic acid such as chromic acid to a coating composition containing a fluorine-containing polymer to form a chemical conversion coating film on a surface of metal for enhancing adhesion of the composition (JP-B-63-2675). However since chromic acid contains hexahydric chromium, it can be said that such a technique is not sufficient in view of safety in food and coating work. Further in case of use of other inorganic acids such as phosphoric acid, there was a problem such that safety of a fluorine-containing resin coating composition is damaged.
Use of a coating composition containing a fluorine-containing resin as a primer, in which heat resistant resins such as polyamideimide, polyimide, polyethersulfone and PEEK and in addition, a metal powder are added instead of the above-mentioned inorganic acid, has been studied (JP-A-6-264000). Inherently there is no compatibility between a fluorine-containing polymer and heat resistant resin. Therefore there arises a phase separation in a coating film, thus easily causing intercoat adhesion failure between the primer and the top coat. Further, film defects such as pin holes and cracks arise easily at the time of processing at high temperature and during use due to difference in heat shrinkage between the fluorine-containing resin and the heat resistant resin or due to lowering of elongation of the coating film by the addition of the heat resistant resin. Also since these heat resistant resins are colored brown by baking, it is difficult to use them for applications requiring white and vivid colors and transparency.
Further, non-sticking property and low friction property which the fluorine-containing resin inherently possesses are lowered.
Also for adhesion of a fluorine-containing resin coating composition to a glass requiring transparency, improvements of the adhesion have been tried by treating the substrate with a silane coupling agent or adding a silicone resin to the fluorine-containing resin coating composition (JP-B-54-42366, JP-A-5-177768, etc.). However thermal resistance and adhesion are insufficient, and separation of film, foaming and coloring arise easily at high temperature.
On the contrary, fluorine-containing coating compositions prepared by copolymerizing a hydrocarbon monomer containing a functional group such as hydroxyl or carboxyl have been discussed. However those coating compositions were originally studied mainly for a purpose of weather resistance, and therefore it is difficult to use them for applications requiring thermal resistance at 200.degree. to 350.degree. C. which is directed by the present invention, and for applications requiring non-sticking property, low friction property, etc.
Namely with respect to a polymer prepared by copolymerizing a hydrocarbon monomer (containing no fluorine) having a functional group, thermal decomposition easily occurs on components of the monomer at the time of processing at high temperature or during use, and thus coating film failure, coloring, foaming, separation, etc. arise, which makes it impossible to attain purposes of coating a fluorine-containing resin.
An object of the present invention is to solve the above-mentioned problems, and to provide a material for fluorine-containing coating composition which maintains excellent characteristics such as thermal resistance, chemical resistance, non-sticking property and low friction property of fluorine-containing polymer and further can have strong adhesion directly to a substrate such as metal, glass, etc., and to provide a method for coating by using the material for coating compositions.